1. Field of the Invention
The present invention generally relates to reliable communications systems and, more particularly, to a spread spectrum communications technique that offers an extra strategy to neutralize the effect of a "partial-band follower" jammer.
2. Description of the Prior Art
Spread spectrum communications offer several advantages in both military and civilian applications. These include low density power spectra for signal hiding, message screening from eavesdroppers and interference rejection. In the case of interference rejection, the interference may be accidental, that is, simply a part of the environment of the communication system, or may be intentional, that is, some form of jamming. There are several types of spread spectrum systems including direct sequence digital systems, frequency hopping systems, time hopping systems, pulsed frequency modulated (or chirp) systems, and various hybrids. Of these, the direct sequence digital systems and the frequency hopping systems are perhaps the more widely implemented.
In a direct sequence digital system, a pseudorandom code generator is used to modulate a frequency modulated carrier. In a frequency hopping system, a coherent local oscillator is made to jump from one frequency to another. The subject invention belongs to the frequency hopping type of spread spectrum communications system. A description of this and other types of spread spectrum communications systems may be found, for example, in Spread Spectrum Systems, 2nd Ed., by Robert C. Dixon, John Wiley & Sons (1984) and Spread Spectrum Communications, Vol. II, by M. K. Simon et al., Computer Science Press (1985).
While spread spectrum communications is a favorite electronic measure to gain immunity from jamming and detection, frequency-hopped spread spectrum systems may be easily jammed by a simple jamming process consisting of several tones or Gaussian noise pulses injected randomly among the frequency bins. This type of jamming, known as "partial-band" jamming, is recognized in the book by M. K. Simon et al., supra, to cause severe degradation in performance compared to other forms of interference. Partial-band jamming is especially damaging in the case when the jamming system (hereinafter "jammer") is sophisticated enough to follow the signal with high probability. There is therefore a need to make frequency-hopped spread spectrum communications more robust in the presence of follower (i.e., multiple tone or multiple Gaussian pulse) jammers.